1. Field of the Invention
The invention relates to a process and apparatus for transferring heat in columns. The invention more specifically relates to shielding heat exchangers in columns from liquid.
2. Description of Related Art
Many industries such as petrochemical, chemical and petroleum refining use columns for separating mixtures. Such columns are typically cylindrical, vertically orientated vessels wherein rising vapor and descending liquid come into contact, transfer components, separate, and pass respectively towards the top and bottom sections of the column. Frequently, columns contain vapor-liquid contacting devices to enhance the contacting and separation of the vapor and liquid phases. Examples of vapor-liquid contacting devices include trays and packings. Many of these vapor/liquid contacting devices have a wide variety of designs. For example, tray types of vapor/liquid contacting devices include bubble cap, sieve, valve, and multiple downcomer trays which typically allow liquid to descend and vapor to ascend through the tray. Packings include random packings, filled with Raschig rings or Berl saddles for example and structured packings. The specific operating conditions of individual columns may vary significantly in order to accomplish the myriad separations for the vastly different mixtures that are processed. Examples of processes that use such columns include stripping, rectification, and various forms of distillation such as fractional distillation, steam distillation, reactive distillation and distillation in divided wall columns. These processes may be operated in either batch or continuous modes. Reducing the installed and operating costs of the column are common objectives of the design and operation. In many cases, this effort focuses on the equipment and utilities required for the supply and removal of heat from the column.
Commonly, heat is supplied or removed from the column by removing a stream from the column, passing it through a heat exchanger external to the column shell, and returning at least part of the stream thus cooled or heated to the column. For example, overhead vapor may be withdrawn from the top section of the column and passed to an overhead system outside the column shell that comprises a heat exchanger which condenses liquid from the overhead vapor and a means for returning at least a portion of the condensed liquid to the column to provide reflux. The overhead system frequently also comprises a receiver to separate the condensed liquid from uncondensed vapor, a pump to transfer the liquid from the receiver to the column, pipes, and valves. Such heat exchangers are commonly referred to as condensers or partial condensers. In an analogous manner, heat exchangers are commonly used to provide vapor to the column by heating a liquid stream removed from the bottom section of the column. Vapor and liquid streams may be withdrawn from the central section between the top and bottom sections of the column, heated or cooled and returned to the column.
Multiple heat exchangers are used to exchange heat in a step-wise manner. For example, heat is recovered from a vapor phase in a first heat exchanger for use elsewhere, such as to generate steam followed by a second heat exchanger to further cool the remaining vapor phase to meet a temperature requirement, such as for storage. Additional examples are illustrated and discussed in E. Kirschbaum, Distillation and Rectification, 82-85 (Chemical Publishing Co. 1948). This text shows a first heat exchanger located inside the top section of a shell and a second heat exchanger located external to the shell of a still in FIG. 49 on page 80 and of a packed column in FIG. 197 on page 310.
U.S. Pat. Nos. 2,044,372; 4,218,289; and 5,507,356; and DE 198 30 163 A1 all describe the use of various heat exchangers inside columns to at least partially condense vapor in the top section of columns.
U.S. Pat. No. 2,044,372 also describes the use of a vertical submerged condenser between a low pressure section and a high pressure section of a single column.
U.S. Pat. No. 5,507,356 also describes the use of a plate heat exchanger operated as a condenser installed between two packed beds or trays within the column situated within a collecting vessel open at the top.
Various benefits can be obtained by using heat exchangers inside columns compared to locating them outside the column shell. For example, some equipment of the overhead system and associated supporting structure can be eliminated when the condenser is located inside the column. This provides savings in terms of both cost and space. Also, the pressure drop through such an internal heat exchanger can be lower than that of the equivalent external overhead system. The lower pressure drop is an especially important factor when the column is operated at subatmospheric pressure such as when the mixture being processed in the column is heat sensitive.
In many applications using two heat exchangers to cool the vapors in the column, much of the benefit obtained in locating the first exchanger in the column, such as lower pressure drop and reduced costs, is lost by locating the second heat exchanger outside of the column.